Field of the Invention
The present invention relates to a method of driving a vibration actuator, a vibration drive device, and a mechanical apparatus equipped with the vibration drive device, and more particularly to a technique for enhancing the sliding efficiency of the vibration actuator to thereby improve the durability thereof.
Description of the Related Art
A vibration actuator that brings a vibration element and a driven element into pressure contact with each other and excites driving vibration in the vibration element to thereby move the vibration element and the driven element relative to each other has such characteristics that the vibration actuator is capable of generating large low-speed torque and holding its state after driving, with a frictional force. By focusing on these characteristics, for example, in an image pickup apparatus, such as a single-lens reflex camera, the vibration actuator has been put into practical use as a lens driving motor for performing an auto-focus operation. Further, in recent years, the vibration actuator is expected to be applied to other uses. For example, the vibration actuator is expected to be applied to the driving of joints of a robot arm, the driving of a robotic hand for rotation, the driving of a photosensitive drum of an image forming apparatus for rotation, the driving of an X-Y stage within a plane, and so on.
To apply the vibration actuator to these other usages, the vibration actuator is demanded to generate larger torque and achieve higher output, and further, is also strongly demanded to improve the sliding efficiency and improve the durability for suppressing the lowering of driving characteristics with time. To meet these demands, it is necessary to suppress the occurrence of an unnecessary slip between the respective contact portions of the vibration element and the driven element brought into contact, to thereby improve the sliding efficiency. Accordingly, for example, for a vibration actuator using bending vibration (vibration causing displacement in a direction normal to a portion of the driving element in contact with the driven element) and stretching vibration (vibration causing displacement in a direction of driving the driven element) as driving vibration, there has been proposed a technique for reducing a slip occurring between the driven element and the vibration element by generating the stretching vibration as a combined vibration of n-th-order vibration and 3n-th-order vibration (see Japanese Patent Laid-Open Publication No. 2007-202227).
However, it is difficult to say that sufficient sliding efficiency can be obtained by the technique described in Japanese Patent Laid-Open Publication No. 2007-202227, and hence further improvement is demanded. Further, the n-th-order vibration and the 3n-th-order vibration which are used in the technique described in Japanese Patent Laid-Open Publication No. 2007-202227 have a large difference between the respective natural frequencies of the vibrations, which causes a problem that the mechanical design (structure design) of the vibration actuator has a significant restriction (a small degree of freedom).